Electrical stimulation therapy may be used for various forms of treatment. For example, stimulation therapy may be provided to address neurological issues such as chronic pain, tremors, and the like. In such an example, an implantable stimulation device is typically located in one location of convenience and is connected to electrical leads that are routed to a stimulation site such as within the brain, the spinal column, within the pelvic region, or elsewhere. The electrical leads include electrodes that interface with the tissue at the stimulation site to deliver the stimulation signals from the stimulation device.
The electrical leads include electrical conductors that span the distance from the stimulation device to the electrodes at the stimulation site, and these electrical conductors are susceptible to receiving undesired currents from induced voltages within the body caused by external conditions because of the conductive loop created from the electrodes back to the conductive case of the stimulation device when configured for unipolar stimulation. High power level deposition may then occur at the electrodes. One particular example of this situation arises when the patient undergoes a magnetic resonance image (MRI) scan. The MRI scan requires that a strong gradient magnetic field be presented to the body. Under conditions normally present when the stimulation device is actively providing unipolar stimulation signals, this magnetic field produced by the MRI machine induces voltage within the body that results in problematic currents on the leads.
The solution has been to deactivate the stimulation therapy during the MRI scan or require the stimulation to be bipolar only and to thereby present high impedances in the case to electrode path. The high impedance effectively attenuates the induced voltage and resulting current in the loop created by the leads to insignificant levels. However, the patient cannot receive unipolar stimulation therapy during this time, and this may be an unacceptable consequence, especially for life sustaining therapies.